a. Field of the Invention
The instant invention relates to filtering of noise in an electrical signal. In particular, the instant invention relates to systems and methods for filtering power line noise from acquired electrophysiological signals, such as ECGs and EGMs.
b. Background Art
Electrophysiology is the study of the electrical properties of and electrical activity in biological cells and tissues, such as the electrical activity that causes a heart to beat. Electrophysiological signals can be acquired and studied in a number of ways, including via electrocardiogram (“ECG”) and electrogram (“EGM”).
Power line interference, which may consist of one or more harmonics, is one source of noise that may contaminate ECG and EGM signals. Notch filters (band-stop filters with narrow stop bands) are often used to filter power line interference out of ECG and EGM signals. For example a notch filter with a stop band between about 59 Hz and about 61 Hz may be used to filter out interference from a 60 Hz power line typically used in the United States. Likewise, a notch filter with a stop band between about 49 Hz and about 51 Hz may be used to filter out interference from a 50 Hz power line typically used in Europe.
When applied to signals having sharp edges, such as ECG and EGM signals, notch filters may cause an undesirable “ringing effect.” Ringing results from sudden changes in the acquired signal (e.g., the ECG or EGM signal) interacting with the notch filter, and typically manifests itself as wild oscillations at the notch filter central frequency (e.g., about 60 Hz in the United States and about 50 Hz in Europe).
Remedies have been suggested to eliminate the ringing effect, including the use of signal segmentation for special treatment of the sharp regions of the ECG or EGM signal and adaptive filters using the power line signal as a reference. Many of these remedies are computationally expensive and are therefore less desirable for use in systems that provide real-time analysis of electrophysiological data.